1. Field of the Invention
Embodiments of the present invention relate, in general, to communications between stations in a wireless network and more particularly to methods and systems for unified contention based period transmission opportunity protection.
2. Relevant Background
Within wireless communication systems, a wireless communication device is normally referred to as a station or STA (e.g., a wireless station). Examples of wireless stations (STAs) include a wide variety of wireless communication devices (e.g., computers including laptop computers, PDAs, cell phones, etc.). In addition, various wireless communication systems can be configured to operate using different means of communication (e.g., ad hoc, peer to peer, etc.). The basic building block of a wireless network is a Basic Service Set (BSS) which is a group of STAs that communicate with each other. Communications take place within the area surrounding these stations called a Basic Service Area (BSA). The BSA is defined by the propagation characteristics of the wireless medium.
An Independent BSS (IBSS) is one in which stations can communicate directly with each other and thus must be within direct communication range. Typically, IBSS networks are composed of small numbers of stations set up for a specific purpose or for a short period of time. One common use is to create a short-lived network to support a single meeting in a conference room. Due to the short duration, small size, and focused purpose, IBSSs are sometimes referred to as ad hoc BSS or an ad hoc network. Such small personalized type of ad hoc networks are also referred to as personal or private IBSS networks (PBSS).
In some wireless communication systems, a PBSS coordinator point (PCP) may operate as a central governing communication device to which and through which various other STAs within the wireless communication systems communicate. For example, the PCP may serve as a coordinator of various other STAs within the wireless communication system or BSS, and it may also serve as a gateway to another network (e.g., a wide area network (WAN), the Internet, etc.). Stations communicate among each other via frames on a communication medium, normally a channel.
In addition to data frames that carry information from higher layers, the IEEE 802.11 standard includes management and control frames that support data transfer. These are the frames on which the PCP operates. The IEEE 802.11 standard defines a distributed coordination function (DCF) (later enhanced and referred to as a hybrid coordination function (HCF)) to provide a best-effort service to the medium access control (MAC) layer of the wireless local area network (WLAN). The standard uses a carrier sense multiple access with collision avoidance (CSMA/CA) protocol that includes a distributed contention-based channel access mechanism referred to as enhanced distributed channel access (EDCA). Another means for medium access used in the 802.11 standard is an optional point coordination function (PCF) which is a centralized polling-based channel access mechanism.
DCF constitutes the fundamental access mechanism of the 802.11 standard. According to DCF, a wireless local area network (WLAN) station must sense the medium before initiating the transmission of a packet. If the station senses the medium as idle for a time interval greater than the distributed inter-frame space (DIFS), then the station transmits a packet. Otherwise, the transmission is deferred and a back-off process starts. Specifically, the station initializes and begins decreasing a timer called a back-off counter. As soon as a back-off counter expires, the station is authorized to access the medium. The initial value of the back-off counter is defined as the back-off window, which is a random time interval, uniformly distributed. Given that collision detection is not possible in the WLAN environment, an acknowledgment is used to notify the sending station that the transmitted frame has been successfully received. The transmission of the acknowledgment is initiated at a time interval equal to the short inter-frame space (SIFS) after the end of the reception of a transmitted frame.
In addition to the basic access mechanism the 802.11 standard includes a protection mechanism for dealing with a hidden node problem. This mechanism is based on the exchange of two short control frames: a request to send (RTS) frame is sent by a potential transmitter to receiver and a clear to send (CTS) frame that is sent from the receiver in response to the RTS frame. The RTS and CTS frames include a duration field to specify the time interval necessary to completely transmit the data frames and the related acknowledgment. Other stations can hear either the sender (RTS frame) or the receiver (CTS frames) in order to refrain from transmitting until the data transmission is complete. The effectiveness of the RTS/CTS mechanism depends upon the length of the packet being “protected”. Usually a hybrid approach is used, where only packets with a size greater than a threshold (called the RTS threshold) are transmitted with the RTS/CTS mechanism.
EDCA distributes access to the communication medium by using a system of user priorities. A combination of priority values and queues assigns each packet an access category before entering the MAC layer. These access categories differentiate between background, best effort, video and voice which are, in turn, factored into individually prioritizing each packet. EDCA is contention based.
A contention-based protocol (CBP) or period is a communications protocol for operating wireless telecommunication equipment that allows many stations to use the same radio channel without pre-coordination. The “listen before talk” operating procedure in IEEE 802.11 is the most well known contention-based protocol. Using a contention based protocol, multiple independent stations can interact without central control since before attempting to transmit each station checks whether the medium is idle. If the medium is not idle, stations defer to each other and employ, as previously described, an orderly exponential back-off algorithm to avoid collisions.
The next generation manufacturing system (NGMS) draft uses CBP in two forms: PCP-centric CBP and distributed CBP. In the PCP-centric CBP EDCA is used to decrease the possibility of collisions. Each transmit opportunity requires multiple transmission slots. Accordingly, the PCP sends out protection frames before each transmission opportunity to prevent collisions but does so in small sectors. This results in a high overhead due to the omnidirectional protection requirement.
Communication using a RTS/CTS protocol is very narrowly tailored so that other stations in other sectors do not receive either the RTS or the CTS. Accordingly they can and do transmit and receive at the same time. This can result in interference. To resolve this interference, the PCP transmits to each station sector-by-sector informing them whether or not they can transmit. In essence the PCP insures that the targeted station is ready to receive the message and that no other stations will transmit during the active state of the data exchange. Thus interference can be totally avoided when the PCP transmits to all stations. In this type of CBP operations the PCP is always active and requires a great deal of overhead. This is commonly referred to as a PCP-centric mechanism.
In a distributed CBP protocol, EDCA is again used to decrease the possibility of collisions. And again, each transmit opportunity utilizes multiple transmission slots. In this type of CBP each source station transmits a RTS message to the destination station. The destination station replies to a selected source station with a CTS message, resulting in lower overhead. This is good for a short transmit opportunity. However, when the PCP is not actively participating in the transmission, omnidirectional protection is unavailable and thus interference can occur.
During the CBP process in which the PCP is not actively involved, a CBP transmission opportunity may need to have omnidirectional protection based on the station's (source and/or destination) environment. Currently PCP power saving and omnidirectional protection are contradictory. Saving power by allowing the PCP to remain inactive can result in significant collisions between source and destination communications. Alternatively maintaining a PCP-centric CBP protocol requires undue overhead and excessive power use.
A need therefore exists to provide omnidirectional collision avoidance protection without PCP direct involvement. These and other challenges of the prior art are addressed by one or more embodiments of the present invention, hereafter described by way of example.